1. Field of the Invention
This invention relates generally to a belt-type continuously variable transmission, in which a speed change ratio is varied continuously by changing effective running radii of a belt applied on a drive pulley and a driven pulley having variable groove widths. More particularly, this invention relates to a control system of the continuously variable transmission.
The disclosure of Japanese Patent Applications No. 2004-34037 filed on Feb. 10, 2004, and No. 2004-363420 filed on Dec. 15, 2004, including specification, drawings and claims is incorporated herein by reference in its entirety.
2. Discussion of the Related Art
In the prior art, it is known to optimize the running state of an engine by arranging a continuously variable transmission on the output side of the engine, and by continuously controlling a gear ratio of the continuously variable transmission. As the continuously variable transmission of this kind, a belt-type continuously variable transmission, and a toroidal type continuously variable transmission are known in the art. In the belt-type continuously variable transmission, a torque is transmitted by a frictional force established at a contacting face between the belt and individual pulleys. Therefore, a contact pressure between the belt and individual pulleys is kept to such a frictional force as corresponding to an input torque, i.e., a torque capacity. Specifically, the contact pressure is secured by clamping the belt by a movable sheave and a fixed sheave so as to exert tension on the belt.
The belt slippage can be avoided by raising the clamping pressure for clamping the belt to increase a transmission torque (i.e., a torque capacity). However, if the clamping pressure is excessively high, transmission efficiency of the power is degraded in the continuously variable transmission, and consumption of the power is increased to establish high oil pressure. For this reason, generally, the clamping pressure is set as low as possible to the extent that the belt slippage does not occur, and varied in accordance with a change in an engine load.
A speed change is carried out in the belt-type continuously variable transmission by changing the groove widths of individual pulleys. Accordingly, a load (i.e., a thrust) is applied to the individual pulleys in an axial direction to carry out the speed change, likewise the load (i.e., the thrust) to establish the clamping pressure. Therefore, according to the prior art, a speed change ratio is arbitrarily set by applying the oil pressure for setting the clamping pressure to the driven pulley, and by applying the oil pressure to the drive pulley which can maintain the clamping force against the oil pressure applied to the driven pulley, while controlling a feeding amount of an operating oil to the drive pulley. Specifically, an oil chamber is provided respectively in the drive and driven pulleys, and the speed change ratio is adjusted steplessly in the belt-type continuously variable transmission by controlling the amount of the operating oil fed to the oil chamber of the drive pulley, and by controlling the oil pressure applied to the oil chamber of the driven pulley.
One example of the belt-type continuously variable transmission of this kind is disclosed in Japanese Patent Laid-Open No. 1994-207658. In the belt-type continuously variable transmission disclosed in Japanese Patent Laid-Open No. 1994-207658, an oil pressure (i.e., a line pressure) relating to a belt tension is feedback-controlled. When the feedback-control is carried out, a delay of the change in the line pressure, in other words, a deviation between a target line pressure and an actual line pressure, is taken into consideration.
In Japanese Patent Laid-Open No. 1997-280361, moreover, there is disclosed a system which carries out a feedback-control of the line pressure by using a target value of an oil pressure (i.e., a line pressure) of a driven pulley which is corrected on the basis of a target value of an oil pressure (i.e., a speed change pressure, or a primary pressure) of a drive pulley at a speed change time.
In Japanese Patent Laid-Open No. 1988-269745, still moreover, there is disclosed a system which alters a control gain of the feedback-control in accordance with the change rate of the speed change ratio, when executing the feedback-control on an oil pressure of the pulley. In Japanese Patent Laid-Open No. 1994-240331, furthermore, there is disclosed a system which carries out a feedback-control of the line pressure when the line pressure is stable, and carries out a feedforward-control of the line pressure when the line pressure is varying.
In the inventions taught by the above-mentioned publications, the oil pressure of the pulley (i.e., the line pressure) in the belt-type continuously variable transmission is appropriately feedback-controlled according to respective constructions. Consequently, convergence of the actual line pressure to the target line pressure, and a control accuracy of the line pressure control at a transient time of the speed change are improved.
Meanwhile, in the control systems for the belt-type continuously variable transmissions of the prior art, the pressure-receiving capacity of the oil chamber in each pulley is varied as a result of displacement of the movable sheave of each pulley at the speed change time. Therefore, a pressure change comes over the oil pressure applied to the oil chamber, and the actual oil pressure is varied temporarily. However, any of the inventions suggested by the above-mentioned publications does not consider the pressure change resulting from the change in the pressure-receiving capacity of the oil chamber at the speed change time. Therefore, the following capability of the actual oil pressure to the target value may be degraded in case of carrying out the feedback control on the oil pressure.
More specifically, in the belt-type continuously variable transmission, the speed change is carried out by varying the groove width of one of the pulleys to which the belt is applied (e.g., the drive-side pulley) so as to change the groove width of another pulley (e.g., the driven-side pulley). Moreover, the clamping pressure of the belt is set to a predetermined pressure by feeding the oil pressure to the oil chamber in the other pulley. For this reason, capacity of the driven-side oil chamber of is changed as a result of the speed change by feeding/discharging the operating oil to/from the oil chamber in the drive-side pulley. Therefore, the change in the pressure-receiving capacity of the driven-side oil chamber impinges on the control of the clamping pressure. In this case, it is also possible to control the oil pressure of the oil chamber for establishing the clamping pressure by the feedforward control. However, the oil pressure is set relatively high so as to improve the following capability of the actual oil pressure to the target value. This may increase power loss and deteriorate durability. In order to solve those problems, the feedback-control on the oil pressure may be carried out. However, since the change in the pressure-receiving capacity impinges on the control accuracy or target following capacity also in this case, a technical improvement is required in order to carry out a pressure control accurately.